Spectrometry is a well known technique used to identify the characteristics of gas, liquid, and solid samples, wherein light is directed at a sample and the light leaving the sample is then picked up by a photosensitive detector to be analyzed for changes in wavelength. These changes provide information regarding the composition of the sample, its chemical bonds, and other features.
It is often desirable to take measurements from multiple samples simultaneously (or nearly so) to increase analysis speed. This can be done by providing multiple sample chambers, and then providing a moving mirror which directs illumination from the light source to each chamber in turn (with the light then being received by one or more detectors). While this arrangement is beneficial, developers have sought to eliminate the moving mirror owing to the burdens of its maintenance, and the sequential illumination of the sample chambers also limits analysis speed since a user must await the results from the later chambers in the sequence.
In one known spectrometry arrangement which is believed to be exemplified by the FTPA2000 200 spectrometer (ABB Inc., Norwalk, Conn., US), multiple fiberoptic cables receive light from a lamp, and each cable illuminates a separate sample chamber containing a sample to be analyzed. Return fiberoptic cables then each receive the light from each sample and provide it to a detector (with one detector per each sample chamber and return cable) to provide analytical measurements. This arrangement can therefore provide truly simultaneous sample measurements while eliminating the moving mirror. However, an arrangement of this nature can suffer from drift in its components; for example, changes in ambient temperature can change factors such as detector sensitivity, the refractive index of the fiberoptic cables, etc., which can in turn affect measurement accuracy. Additionally, such an arrangement is also susceptible to measurement uncertainties owing to differences between the different “channels” used to obtain measurements from the different chambers. Different channels can experience different degrees of drift, and it is also difficult to obtain the “same light” (i.e., the same light flux/intensity) into each of multiple cables arrayed about the light source. Beamsplitters (e.g., dichroic mirrors, prisms, etc.) can be used to divide the light from a light source into a number of different beams of approximately equal intensity to supply the input cables, but here too drift, imperfections, etc. limit the ability to exactly match light input to the different input cables.
A similar arrangement, which is believed to be exemplified by the InfraSpec NR800 spectrometer (Yokogawa Electric Corporation, Tokyo, JP), has multiple fiberoptic input cables extending from a light source, with each illuminating a separate sample chamber. Each input cable is provided with a beamsplitter whereby its transmitted light is divided into two portions, one illuminating its sample chamber (and subsequently a sample detector) and one illuminating a reference detector. Comparison of the measurements from the sample and reference detectors beneficially allows the sample detector measurements to be at least partially compensated for drift. However, this arrangement still has the disadvantages that the beamsplitter still may not provide the same light to the sample and to the reference detector, and additionally the input cables may not each receive and provide the same light from the light source.
It would therefore be useful to have available additional spectrometer arrangements which allow simultaneous (or nearly so) measurements from multiple samples, while at the same time minimizing (or compensating for) drift within and between the channels used to measure each sample.